In the following, the invention will be described especially on the basis of nitrate sensors and SAC sensors. The invention is, however, not limited thereto, but, instead, relates to photometric sensors in general. Measuring devices suitable for determining the corresponding process variables are manufactured and sold by the group of firms, Endress+Hauser, in a large multiplicity of variants, for example, under the designation “Viomax CAS51D”.
The acronym “SAC” stands for spectral absorption coefficient. A “SAC sensor” herein means a sensor, which determines the spectral absorption coefficient by means of absorption of measuring radiation by certain substances of a medium. Especially thereby, the total organic carbon (TOC) contributing to the SAC value and the chemical oxygen demand (COD) can be determined. From the SAC value, the TOC value, respectively the COD value, can be deduced.
In the case of SAC sensors and nitrate sensors, as a rule, the measured value is determined via an absorption measurement at a measuring wavelength and at a reference wavelength. These wavelengths are located in the UV and UV-VIS regions. A possible measuring wavelength and reference wavelength combination for nitrate is 214 nm and 254 nm and for SAC, 254 nm and 550 nm. Other combinations are, however, possible. The measuring principle will be explained briefly based on a nitrate sensor. The section of the sensor immersed in the medium has a gap (the cuvette gap), into which the medium can penetrate. The gap is traversed by a measuring beam (having a measuring wavelength) and a reference beam (having a reference wavelength). Nitrate ions contained in the medium, and therewith in the gap, absorb the light in the region of the measuring wavelength of 214 nm in proportion to their concentration, while the UV light in the reference channel at 254 nm remains almost unchanged. Used as measurement result is the ratio between the reference channel and the measurement channel. This ratio is converted into the nitrate concentration based on a stored calibration curve.
The calibration curve is obtained earlier via suitable calibration means. Frequently, certain standard liquids are used. Typical calibration means for nitrate sensors are solutions of nitrate hydrogen phthalate (NHP) or potassium hydrogen phthalate (KHP). These standard solutions have, however, disadvantages as follows: Limited storability, multiple use doubtful because of possible standard solution contamination, and expensive for the sensor operator. Due to the frequently to be performed calibrations/adjustments and function testings with standards, these sensors represent a significant burden for the sensor operator.
As an alternative to liquid solutions, solid standards are a possibility. Solid standards can be stored longer, provided that materials that age are not used. Additionally, their handling is less of a problem.
German Patent DE 10 2009 028 254 A1 discloses a solid standard, which has in a certain configuration a similar spectral behavior as KHP, and, thus, simulates a certain concentration of KHP. Other concentrations of KHP or other calibration means cannot be simulated by this disclosed solid standard. The standard is additionally fixedly arranged in the device.
A contamination check, and therewith a function review, is implemented, for instance, by one or more control filters installed in the apparatus. These control filters have a defined absorption and weaken the light by a certain amount. Fouling is present when a difference from the expected measured value is detected.
There are systems with a plurality of filters, which are introduced alternately or simultaneously into the optical path, i.e., for example, into the earlier described gap, in order to perform a verification or calibration. U.S. Pat. No. 6,977,365 B1 shows such filters. The filters are, in such case, embodied as differently gray filters, so that, depending on filter set, a weakening can be detected compared with measuring without filtering. One obtains thus a number of (filter-)measured values compared with measuring without filtering, however, without any wavelength dependent information. In this way, indeed, the slope of the calibration curve can be determined and, in given cases, readjusted, but a determining of the y-intercept is not possible.